This invention relates to a method to improve the recovery of steamflooding. More particularly, the method comprises injection of steam or production of hydrocarbons by injection or production through alternating rows of wells.
Numerous techniques have been suggested to enhance the recovery of hydrocarbons from underground formations. Waterflooding and steamflooding have proven to be the most successful of these recovery techniques employed commercially. However, these techniques may still leave up to 60% to 70% of the original hydrocarbons in place, depending on the characteristics of the formation and the oil.
In conventional steamflooding, steam is injected into the formation and fluids are produced from the formation until the ratio of hydrocarbons produced to steam injected is so low as to make the flood no longer economical. In a typical steamflood, after steam breaks through to the producing wells, the proportion of hydrocarbons produced relative to injected steam steadily decreases. Steam breakthrough at the production well generally indicates that a flowpath of steam from the injection wells to the production wells has formed. Once formed, such a flowpath will generally be followed by later injected steam, thereby diminishing the ability of the later injected steam to reach and displace hydrocarbons in portions of the formation not adjacent to the flowpath.
Various methods have been proposed to overcome the disadvantages of such steam channelling and override in a steamflood. These methods include surfactants, steam foams, gels, additional wells, fracturing, and other techniques. U.S. Pat. No. 3,385,360 discloses a cyclic steam drive wherein the rate of steam injection is reduced in the low cycle to no more than 60% of the initial injection. The patent also states that the quality and temperature of the steam may be varied, although there is no disclosure of interruption of steam injection, or injection or production in alternating rows.
U.S. Pat. No. 3,480,081 discloses pressure pulsing of oil production, wherein one embodiment describes the injection of steam during a pressurizing step while other wells are produced during a depressurizing step. U.S. Pat. No. 3,273,640 describes a complicated process for the extraction of shale oil from rock involving pressurization with steam and an intermittent relief of the pressure to encourage flow from voids and edges of the shale formation.
Pressurization and production interruptions are disclosed in numerous tar sand and bitumen references. But because of the different structure of tar sands and bitumen, these processes encourage channelling, the exact opposite goal of the instant invention. Thus, such references to tar sand shale oil processes are not relevant to the present invention. One of these, J. A. Dilabough, et al., "Recovering Bitumen From Peace River Deposits," Oil & Gas Journal, Nov. 11, 1974, pp. 186-198 discloses a cyclic process for bitumen with repeated depressurizing steps starting from six months after injection of various fluids including steam and lasting for up to 11/2 years.
U.S. Pat. Nos. 3,354,954 and 4,733,726 disclose the interrupted operation of a production well in steamflooding. T.M. Doscher, et al., "The Anticipated Effect of Diurnal Injection on Steamdrive Efficiency," Journal of Petroleum Technology, Aug. 1982, pp. 1814-1816 discusses a study of the performance of steam drives when cyclic steam injection is employed.
A variation on the WAG process called water-alternating-steam process (WASP) is disclosed in Hong, K.C. et al., "Water-Alternating-Steam Process Improves Project Economics at West Coalinga Field," CIM/SPE Paper No. 90-84, presented at the International Technical Meeting Hosted by the Petroleum Society of CIM and the SPE in Calgary, Alberta, Jun. 10-13, 1990. In this process steam injection is alternated with water injection.
A discussion on various oscillating injection and production methods for steamflooding can be found in a paper authored by the instant inventor. Please see, Chu, C., "Oscillating Injection-Production Schemes for Steamflooding Oil Reservoirs," SPE Paper No. 21797, presented at the Western Regional Meeting of the SPE in Long Beach, California, Mar. 20-22, 1991. The paper contains no discussion of staggered scheduling of injection or production into alternate rows of wells.
Other methods involving interrupted injection of steam with or without other fluids, and interrupted production of fluids, many for tar sands and bitumen or with the use of infill wells, are disclosed in a number of references. Some of these references are U.S. Pat. Nos. 4,088,188; 4,124,071; 4,160,481; 4,166,501; 4,166,502; 4,166,503; 4,166,504; 4,175,618; 4,177,752; 4,296,969; 4,431,056; 4,450,911; 4,465,137; 4,488,600; 4,491,180; 4,495,994; 4,515,215; 4,597,443; 4,612,990; and 4,700,779.
There continues to be a need for improving steamflood recoveries without significantly increasing the cost of the steamflood and without damaging the formation.